Process for cleaning the waste gas from drying processes using heated gas

ABSTRACT

The waste gas accumulating in the drying of products containing volatile ingredients is cooled, the condensate obtained is subjected to crossflow membrane filtration and part of the cleaned waste gas is returned to the drying process. The concentrate obtained is put to a material, thermal or other use. The both effective and economic process is suitable for eliminating the aerosols, particularly the organic aerosols, present in the waste gas.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

This invention relates to a process for cleaning the waste gas fromdrying processes using heated gas for products containing volatileingredients, more particularly for cleaning the waste gas accumulatingin the spray drying of surfactant-containing materials, such asdetergent slurries.

More particularly, the invention relates to, but is not confined to, theindustrial production of surfactants and detergents by drying of aslurry in spray drying towers. The slurry is sprayed through nozzlesinto the drying zone and dried with the hot gas stream flowing inco-current or countercurrent. The drying gas used is generally air takenin from the surrounding atmosphere which is mixed with the waste gas ofa burner and thus heated. Accordingly, the drying gas contains the waterfrom the ambient air and the water formed during the combustion process.

However, the invention is not confined to a particular type of dryinggas. It may consist of air or other gases, for example superheatedsteam.

The drying of products containing volatile ingredients removes not onlywater, but also relatively large or relatively small amounts of thevolatile components, depending on the drying temperature. In theproduction of detergents by drying of a water-containing paste, theslurry, the volatile components are mainly organic substances, forexample nonionic surfactants in the form of fatty alcohols with lowdegrees of ethoxylation and unsulfonated fatty alcohols in fatty alcoholsulfates. They are present in gaseous form or in the form of extremelyfine condensation aerosols in the waste gas of the installations usedfor drying, for example spray dryers. The aerosols frequently haveconcentrations of the order of 100 ppm in the waste gas. On account oftheir extremely small particle size, they cannot be separated to asatisfactory extent by the textile filters or cyclones normally used toclean waste gases from drying installations and, accordingly, pass tothe outside with the waste gas stream. Separation of the aerosols bytextile filters, even to a small extent, is actually a disadvantagebecause the filters become blocked and have to be changed after arelatively short time. The waste gas emission produced by the aerosols,which is brought about above all by the steam volatility of certaincomponents of the slurry, is known as "pluming".

DISCUSSION OF RELATED ART

Numerous solutions have been proposed in the prior art for reducingunwanted pluming. The temperature of the drying gas can be reduced, thefinal water content of the dried product can be kept high and, inaddition, drying can be carried out on the countercurrent principle.However, these variants reduce the efficiency of the dryinginstallation, affect product quality and limit the process controlpossibilities. Another known method of reducing pluming is to modify thecomposition of the slurry. The disadvantage of this is that the optimalformulation has to be changed.

It has also been proposed to condense the gaseous organic constituentsin a washer-cooler. Unfortunately, washing the waste gas in this way isuneconomical on account of the high throughput of drying air which is inexcess of 100,000 m³ /h. In addition, the environment problem is merelyshifted from the waste air to the wastewater.

It is generally not possible to use a countercurrent ionization washerfor cleaning waste air in view of the high moisture content.

Another known method of eliminating aerosols is to burn the waste air.However, this involves the additional consumption of energy which can behigher than the energy consumption of the actual drying process.

To summarize, it may be said that the known methods for reducing oreliminating pluming are complicated and expensive.

Accordingly, the problem addressed by the present invention was toprovide an effective and economic process of the type mentioned at thebeginning for eliminating the aerosols, particularly the organicaerosols, present in the waste gas.

DESCRIPTION OF THE INVENTION

According to the invention, the solution to this problem ischaracterized in that the waste gas is cooled, the condensate obtainedis subjected to crossflow membrane filtration, part of the cleaned wastegas is returned to the drying process and the concentrate obtained isput to a material, thermal or other use.

According to the invention, the waste gas is cooled within the dryinginstallation. This is deliberately avoided in known processes to avoidproblems with condensate separated from the waste gas. Cooling has onlyever been carried out to recover the waste heat. In the processaccording to the invention, cooling of the waste gas leads to partialcondensation of and an increase in the aerosols which are separated fromthe waste gas in following aerosol separators. In the case of organicwater-insoluble aerosols, an oil-in-water emulsion generally accumulatesin the separators. It is subsequently treated by the membrane processmentioned. Only a small quantity of concentrate is left as residue.

The cleaned waste gas, which only contains extremely small quantities ofaerosols, is returned to the drying process. There are no unwantedproblems with carbon dioxide, which can react undesirably for examplewith basic ingredients of the material to be dried. This is because alarge part of the carbon dioxide dissolves in the aqueous phase of thecondensate and is removed with it from the waste gas stream.

In one advantageous embodiment of the process according to theinvention, water is finely sprayed in to cool the waste gas. Theaccompanying saturation with steam is known as "quenching" and must notbe confused with the washing of a waste gas which would be uneconomicalin view of the normally relatively large quantities of waste gas of theorder of 100,000 m³ /h or more.

The waste gas may be cooled in various ways. Thus, in a first embodimentof the invention, the waste gas is cooled by surface condensers. Thecondensate accumulating, which again is generally an emulsion, isafter-treated in the same way as described above.

It has proved to be particularly favorable to cool the waste gas by atleast 40° C.

If the waste gas is cleaned by passage through textile filters, as isstandard practice in the prior art, clogging of the filter can beprevented by cooling the waste gas only after it has passed through thetextile filter because the aerosols, which are still very small beforecooling, readily pass through the filter.

The waste gas contains only a small amount of aerosols after cooling andseparation of the condensate. In one advantageous embodiment of theinvention, the aerosol component of the waste gas can be reducedparticularly effectively by adding fresh air to part of the cleanedwaste gas and delivering the mixture to the burner of the dryinginstallation. In this case, the small residues of organic aerosolconstituents still present in the cleaned waste gas burn substantiallycompletely. A high percentage of the recycled waste gas in thecombustion air leads to a particularly favorable cleaning result. Thispercentage may be up to about 90% of the combustion air.

The recycling of the uncleaned waste gas to the drying zone is basicallyknown as "recirculating air drying". According to the present invention,however, the waste gas is only recycled after the removal of aerosolconstituents.

The condensate accumulating in the process according to the invention isworked up by crossflow membrane filtration. Organic or inorganicmicrofiltration or ultrafiltration membranes are generally suitable,depending on the composition of the emulsion. It is of particularadvantage, especially from the economic point of view, to use amicrofiltration membrane.

The permeate obtained, which is also called the filtrate, has such lowwastewater values that it can generally be safely discharged into themain drainage system. However, if greater purification of the permeateis required, it is proposed to purify the permeate obtained from thecrossflow membrane filtration by nanofiltration or reverse osmosis.Alternatively or in addition, the permeate may be after-treated with anoxidizing agent, for example peroxide, UV light or ozonization. Anotherpossibility is to subject the permeate to biological purification. Thebiological purification stage may be a central sewage treatment plant ora decentralized bioreactor.

The concentrate (retentate) from the crossflow membrane filtration ispreferably put to a material use. If this is not possible, it isdisposed of, for example by burning.

The process according to the invention may be used on the one hand inclosed drying installations operated, for example, with superheatedsteam or with ambient air. After the condensation stage, there remainsonly a relatively small and non-condensible quantity of waste air which,in this case, is completely or substantially completely recycled.Although the waste air emission plays only a small part in this case,the aerosol constituents of the circulated drying gas have to becontinuously removed to stop any increase in concentration. Theinvention affords a particularly economic possibility in this regard.Accordingly, it is proposed to apply the process to waste gas fromdrying installations using circulated drying gas.

The advantages of the invention become particularly clear when theprocess is applied to waste gas issuing from open drying installations.Particularly serious waste air emission problems have been encounteredin such installations in the prior art, but are solved economically andeffectively in accordance with the present invention.

DESCRIPTION OF THE DRAWING

One embodiment of the invention is described by way of example in thefollowing with reference to the accompanying drawings, wherein:

FIG. 1 is a flow chart of an industrial embodiment of the invention.

FIG. 2 is a graph illustrating test results.

A spray drying process operated with the waste air treatment accordingto the invention is illustrated in FIG. 1. For spray drying in a spraydrying tower 1 operated with air as the drying gas, a detergent slurryis delivered through a pipe 2 and sprayed through nozzles into thedrying zone of the spray drying tower 1. The waste air is cooled in asurface condenser 3 or by spraying in water, most of the aerosols beingseparated. The emulsion obtained from the condensate separator 12consists of the water formed by condensation and organic substances. Itis delivered to a membrane filtration unit 4 from which a concentrate(retentate) 6 and a filtrate (permeate) 7 is obtained.

Part of the cleaned waste air 5 is passed to the outside. The other partis added through a pipe 13 to the fresh air 8 of the burner 9 which istaken in by a blower 14. The residues of organic aerosols still presentin this part of the cleaned waste air burn together with the combustiblegas 10 in the burner 9. The dried product may be removed through adischarge pipe 11 at the cone of the spray drying tower 1.

EXAMPLES

FIG. 2 shows the results of the membrane filtration of a correspondingcondensate. The emulsion was very stable and could not be broken in acentrifuge.

A ceramic membrane with a pore width of 0.14 micrometer ("Carbosep M14",a product of Rhone Poulenc) was used in the test. The membrane stage wasoperated in batches, i.e. the concentrate was returned to the storagevessel for starting material. The trend of the TOC values in g/l of theconcentrate and permeate over the duration of the test in hours is shownin FIG. 2. After a test period of 6 hours, around 25 kg of concentratewith a TOC value of around 20 g/l were obtained from around 900 kg ofemulsion with a TOC value of about 1 g/l. Despite this highconcentration, the TOC value of the permeate was less than 0.32 g/l.

This shows that the wastewater accumulating in the cleaning of waste airin accordance with the invention can be simply and inexpensivelyconditioned and that a permeate suitable for discharge into the maindrainage system is obtained. In addition, this ensures that the solutionto the waste air problem does not give rise to a wastewater problem.

List of Reference Numerals

1. Spray drying tower

2. Pipe

3. Surface condenser

4. Membrane filtration unit

5. Cleaned waste air

6. Concentrate (retentate)

7. Filtrate (permeate)

8. Fresh air

9. Burner

10. Combustible gas

11. Discharge pipe

12. Condensate separator

13. Pipe

14. Blower

What is claimed is:
 1. The process of cleaning a waste gas containingvolatile components obtained from a drying process using a heated gas todry a composition containing volatile components, comprising coolingsaid waste gas to form a condensate whereby at least a part of saidvolatile components are separated from said waste gas and become part ofsaid condensate, subjecting said condensate to crossflow membranefiltration to obtain a retentate and a filtrate, returning part of saidwaste gas to the drying process, and disposing of the retentate.
 2. Aprocess as in claim 1 wherein said cooling is conducted by spraying saidwaste gas with water.
 3. A process as in claim 1 wherein said cooling isconducted by contacting said waste gas with a surface condenser.
 4. Aprocess as in claim 1 wherein the waste gas to form a condensate iscooled by at least 40° C.
 5. A process as in claim 1 including cleaningthe waste gas by passing the waste gas through textile filters, and thencooling the waste gas.
 6. A process as in claim 1 wherein after formingsaid condensate, the further step of adding fresh air to part of thewaste gas and delivering the mixture to a burner means of a dryingapparatus.
 7. A process as in claim 1 wherein said crossflow membranefiltration is conducted with a microfiltration membrane.
 8. A process asin claim 1 including purifying said filtrate by nanofiltration or byreverse osmosis.
 9. A process as in claim 1 including treating saidfiltrate with an oxidizing agent.
 10. A process as in claim 1 includingbiologically-purifying said filtrate.
 11. A process as in claim 1wherein said waste gas is obtained from a drying process using a heatedgas.
 12. A process as in claim 1 wherein said waste gas is obtained froman open drying apparatus.
 13. A process as in claim 1 wherein saidcomposition comprises a surfactant-containing composition .
 14. Aprocess as in claim 13 wherein said surfactant-containing compositioncomprises a detergent slurry.
 15. A process as in claim 1 wherein saidheated gas comprises heated air.
 16. A process as in claim 1 whereinsaid heated gas comprises superheated steam.